The present invention relates to ductile cast iron which has an improved toughness and particularly superplasticity at the temperature ranging from the eutectoid temperature to about 50.degree. C high above that temperature and so has a better plastic processing ability, and the methods of heat treatment to obtain the said iron.
Cast iron has made a remarkable progress by the invention of spheroidal graphite cast iron, i.e. ductile cast iron - thereafter will be called as s.g. iron -. However, it has not attained yet to the steel level in points of tensile ductility and impact strength, although to improve these properties of s.g. iron, several attempts such as graphite nodule refinement and the addition of special elements have been made. Further, these processes need some special melting process and have a disadvantage of using expensive raw materials.
In addition, cast iron has little use as material for the plastic processing because of its poor plasticity.
Therefore, if the plastic deformability could be improved without spoiling other good properties of cast iron, the use of s.g. iron may be extended more widely.